There are many apps already on store that can do this job. 
If you are looking it for study purpose here is the XML snippet:  https://pastebin.com/aVmZDGNd

here how it will look:  https://imgur.com/gallery/7EJJHtP

Try this https://stackoverflow.com/questions/27731799/how-can-i-remove-powered-by-odoo-on-odoo-8-0-openerp/27731929#27731929